The spinning of the nuclei of certain atoms such as .sup.1 H, .sup.13 C, .sup.15 N, .sup.19 F, .sup.23 Na, and .sup.31 p generates a magnetic moment along the axis of spin. When placed in an external magnetic field, alignment with, or against, the magnetic field may result. Since alignment with the magnetic field is more stable, energy must be absorbed to excite the nucleus to the less stable alignment against the field. The frequency of the radiation energy required to excite a given nucleus is proportional to the strength of the external magnetic field; the stronger the magnetic field, the higher the frequency of the radiation required. When such nuclei return to their lower energy state, absorbed radiation is emitted, and a signal may be detected. Various analytical techniques utilize these magnetic resonance principles.
For example, nuclear magnetic resonance (NMR) spectroscopy has been employed to analyze the structure of chemical compounds. Generally, in this technique, the radiation frequency is kept constant, and the magnetic field strength varied. At some value of applied magnetic field strength, which value is characteristic of the type of nucleus and the environment in which it is found, the energy required to excite the nucleus matches the energy of the radiation, absorption occurs, and a signal may be observed. The number, positions and intensities of the signals obtained while varying the magnetic field strength are recorded as a nuclear magnetic resonance spectrum which provides detailed information on molecular structure. It has been reported by Shulman et al. that NMR spectroscopy has been employed in certain cases both in vitro and in vivo. (Shulman et al., "Nuclear Magnetic Resonance Spectroscopy in Diagnostic and Investigative Medicine", J. Clin. Invest., Vol. 74, 1127-1131 (1984)). NMR spectroscopy, however, while capable of providing extensive information on compounds assayed, requires high field homogeneity across the sample in order to obtain accurate spectra, and a means of varying the magnetic field.
Other NMR techniques include magnetic resonance imaging (MRI), which has been used to study morphology in vivo. Generally, in this technique, the parameters governing the intensity of signals emitted by protons, such as the longitudinal relaxation (T.sub.1) and transverse relaxation (T.sub.2) times, are measured across a subject. Measurements are obtained by applying a magnetic field gradient, that is, a magnetic field the strength of which varies across the subject, and applying pulsed radiation energy. The magnetic field gradient allows data to be obtained which can be converted into two and three dimensional images. Especially when used in conjunction with compounds enhancing contrast such as by shortening T.sub.1 ("contrast agents"), MRI provides clinically useful data, such as data allowing the detection of morphological abnormalities. To enable images of the subject to be obtained, however, MRI equipment is generally large and cumbersome, so that the technique is unsuitable for the determination of the concentration of xenobiotic compounds in settings such as a patient's room or a physician's office.
In vitro measurement of NMR T.sub.1 relaxation times for the determination of glomerular filtration rate is described in Choyke et al., Kidney International, Vol. 41 (June 1992). In vitro testing, however, requires the sampling of body fluids such as blood and urine. Withdrawal and testing of such fluids adds time and expense to the evaluation of a patient, and is particularly undesirable from the standpoint of handling and sanitation.